Hypothermia is a condition in which body temperature is at a level lower than normal body temperature. Hypothermia can be endogenous or exogenous. Endogenous hypothermia occurs because heat produced by normal metabolism is reduced due to conditions such as hypoperfusion of tissues which limit the delivery of oxygen and nutrients necessary for cells to metabolize and thus produce heat. Exogenous hypothermia occurs when external factors create a temperature gradient which promotes more heat loss or transfer from the body to the environment than the metabolism can compensate for. The external factors can include the ambient environment around the body or it can include the provision of hypothermic stimuli into the body such as cold intravenous fluids or intravascular devices which create the heating gradient. Hyperthemia follows these same general principals but in a reverse fashion.
Therapeutic hypothermia (TH) is now a well-developed technique used in certain surgeries where blood flow to the brain may be jeopardized and in the post resuscitation care of the victim of cardiac arrest. In both settings, this is done to help preserve or improve neurologic function. Hypothermia can be induced by both external and internal means, as well as combinations of external and internal means. For example, cooling pads applied to the body surface can be used to lower body temperature by cooling from the exterior surface of the patient. Cooling can also be achieved by immersing the patient in cool water or exposing them to cool air, for example. Often times it is desirable to cool the body very quickly, and this can be achieved by an internal means referred to as endovascular cooling. In operation, heat is removed from the blood using a catheter (an endovascular cooling catheter) such that blood flowing across the surface of the cooled catheter transfers heat, and blood of reduced temperature then flows throughout the body to cool the body and its vital organs. Providing therapeutic warming is simply done the same way except that the external or internal means transfers heat into the body.
Despite the routine use of TH, little is known concerning what the optimal temperature is that will result in the most favorable outcome. Currently, most devices allow targeting of a temperature and utilize a temperature feedback control scheme to permit closed loop control of cooling and heating. For this, endovascular temperature modulating catheters are placed in either the inferior or superior vena cava.
U.S. Patent Publication 2009/0131835 to Voorhees describes a patient temperature response control system used in TH. In operation, the Voorhees system utilizes a variety of sensors (motion sensors, vasoconstriction sensors, electomusculature sensors, carbon dioxide sensors, and blood oxygen sensors) to monitor the patient to detect a physiologic response of a patient (such as shivering) to a change in temperature of the patient, and to control temperature and the delivery of anti-shivering medication.
U.S. Patent Publication 2009/0043366 to Dae describes using endovascular cooling to treat septic shock and other disorders. Dae shows measurement of a variety of patient parameters with manipulation of patient temperature.
Neither Voorhees nor Dae permit a true optimization because their feedback loops are designed solely for manipulation of temperature and not other variables by a single system. Thus, depending on a number of factors, the patient may be cooled inappropriately. Further, with these devices the patient cannot be simultaneously resuscitated in a closed loop fashion.
Such ability is critical to ensure the appropriate application of cooling or warming to the body. For example, providing hypothermia to a patient who is not well resuscitated and is hypoperfused may cause significant life threatening complications such as coagulopathy.